Large deformable elastic braces (LDEBs) are devices which do never yield subject to large deformation under great earthquakes. In structures with LDEBs, elastic restoring force by LDEBs can improve seismic response under great earthquakes. In previous works, the effectiveness of LDEBs for the steel structures was confirmed by experimental tests and seismic response analyses. Here, the topology of LDEBs is determined by an optimization method, and seismic response analyses of RC portal frames with LDEBs are conducted. The effectiveness of LDEBs for the RC structures is discussed. RC portal frame are designed by Japanese seismic design code. LDEBs are equipped with the frame as knee braces. Seismic responses of not only the frame with LDEBs but also that without LDEBs are computed by dynamic nonlinear analysis software. In the analysis LDEBs are regarded as elastic bar elements. An input earthquake is JMA Kobe NS wave (1995 Kobe earthquakes). Hysteresis of story shear force and story drift, axial deformation of LDEBs, maximum and residual story drift are investigated. It is observed from computational results that LDEBs show remarkable improvements on maximum and residual story drifts of RC portal frame under a very large amplitude earthquake such as 1995 Kobe earthquake in Japan.